Catalytic and Enantioselective Route to Medium-Ring Heterocycles. Asymmetric Zirconium-Catalyzed Ethylmagnesation of Seven- and Eight-Membered Rings

Abstract

A variety of medium-ring heterocycles, prepared efficiently by the Ru-catalyzed diene metathesis method, undergo asymmetric catalytic ethylmagnesation to afford nonracemic unsaturated alcohols and amides in excellent enantiomeric purity (>98% ee). Noteworthy features of these studies are as follows: (i) Eight-membered unsaturated tosyl amides are readily prepared by transition-metal-catalyzed metathesis. (ii) With six-, seven-, and eight-membered N-containing substrates the presence of an electron-withdrawing Ts unit is required for efficient carbomagnesation (corresponding alkylamines are inert). Chiral medium-ring heterocycles are resolved by the Zr-catalyzed C−C bond-forming reaction to afford recovered starting materials in up to >99% ee. The kinetic resolution data indicate that simple steric models can reliably predict the sense of the asymmetric induction in the asymmetric carbomagnesation or kinetic resolution. However, experimental results presented herein also illustrate that the observed levels of enantioselectivity cannot be predicted on the basis of such paradigms.